The present invention relates to battery cell tray assemblies for power applications involving deep discharge duty cycles. More particularly, the invention relates to a battery cell tray assembly having a battery housing sized for holding and compressing multiple stacks of valve-regulated lead-acid battery cells arranged in a horizontal position.
For quite some time it has been known that lead-acid batteries are particularly suitable for applications involving xe2x80x9cdeep dischargexe2x80x9d duty cycles. The term xe2x80x9cdeep dischargexe2x80x9d refers to the extent to which a battery is discharged during service before being recharged. By way of counter example, a shallow discharge application is one such as starting an automobile engine wherein the extent of discharge for each use is relatively small compared to the total battery capacity. Moreover, the discharge is followed soon after by recharging. Over a large number of repeated cycles very little of the battery capacity is used prior to recharging.
Conversely, deep discharge duty cycles are characterized by drawing a substantial portion of the battery capacity before the battery is recharged. Typical applications that require deep cycle capability include Class 1 electric rider trucks, Class 2 electric narrow aisle trucks and Class 3 electric hand trucks. Desirably, batteries installed in these types of vehicles must deliver a number of discharges during a year that may number in the hundreds. The cycle life of batteries used in these applications typically can range from 500-2000 total cycles so that the battery lasts a number of years before it needs to be replaced.
Until recently, only lead-acid batteries of the flooded variety have been utilized for the aforementioned deep discharge applications. Flooded lead-acid batteries are designed to have an excess of electrolyte that floods the cell container, completely saturating the plate group and extending into the head space above the plate group to provide a reservoir. The electrolyte reservoir is necessary because as the battery is charged, water in the electrolyte is electrolyzed into oxygen and hydrogen gases, which escape from the cell and deplete the electrolyte volume. To make up for the loss of electrolyte, water must be periodically reintroduced into the cell, or the reservoir must be made large enough to compensate for the expected loss over the life of the battery.
More recently, valve-regulated lead-acid (VRLA) batteries have been introduced that are suitable for deep discharge applications. VRLA batteries rely upon internal gas recombination to minimize electrolyte loss over the life of the battery, thereby eliminating the need for re-watering. Internal gas recombination is achieved by allowing oxygen generated at the positive electrode to diffuse to the negative electrode, where it recombines to form water and also suppresses the evolution of hydrogen. The diffusion of oxygen is facilitated by providing a matrix that has electrolyte-free pathways. The recombination process is further enhanced by sealing the cell with a mechanical valve to keep the oxygen from escaping so it has greater opportunity for recombination. The valve is designed to regulate the pressure of the cell at a predetermined level, hence the term, xe2x80x9cvalve-regulatedxe2x80x9d.
There are two commercially available technologies for achieving the enhanced oxygen diffusion. One technology makes use of a gelled electrolyte. In gel technology, the electrolyte is immobilized by introducing a gelling agent such as fumed silica. Gas channels form in the gel matrix in the early stages of the cell""s life as water is lost via electrolysis. Once the gas channels are formed, further water loss is minimized by the recombination process. Unlike a fibrous matrix, the gel matrix keeps the electrolyte immobilized and there is little bulk movement.
The other technology for enhancing oxygen diffusion makes use of a fibrous material separator between the electrodes. A widely used material for this purpose is an absorbed glass mat (AGM). The AGM is a nonwoven fabric comprised of glass micro-fibers that retain the electrolyte by capillary action, but also provide gas spaces as long as the matrix is not fully saturated with electrolyte. The electrolyte is still free to move within the matrix, but is more confined than in a flooded cell. Another fibrous material gaining acceptance is a non-woven mat constructed from a polymeric component such as polypropylene or polyethylene.
One important difference between the fibrous mat and gel technologies, stemming from the degree of electrolyte mobility, is the effect of cell orientation on cycle life. With fibrous mat technologies, particularly when dealing with cells over about 14 inches tall, it has been discovered that the cycle life in deep discharge applications can be significantly improved by arranging the cells so that the longitudinal axis of the cell lies in a horizontal plane rather than a vertical plane as is customary. With gel technology, there is little difference in deep cycle life when cells are arranged horizontally or vertically. Thus, to achieve maximum cycle life with fibrous mat constructions, it is desirable to orient the fibrous mat cells horizontally, but it is not necessary to orient gel cells horizontally. Presumably, this effect can be explained by stratification of the electrolyte in fibrous mat cells when subjected to deep discharge cycling due to the higher degree of mobility compared with gel technology. The stratification results in reduced discharge capacity and can only be reversed with great difficulty.
The benefits of valve-regulated, lead-acid cell batteries of the fibrous mat variety and cell arrangements for deep discharge applications are known in the art. Although there are applications that take advantage of horizontal cell orientation, they are not without their shortcomings. There is known a xe2x80x9cmonoblocxe2x80x9d battery wherein individual cells are not individually formed and enclosed within separate containers. Rather, they are formed by installing plates in a housing having separate cell compartments, and filling each compartment with acid. Individual cell compartments are defined within the battery case between partitions that are sealed to the battery case walls. A significant disadvantage of this approach is the lack of flexibility to adapt the battery configuration to battery compartments of different sizes. That is, a xe2x80x9cmonoblocxe2x80x9d battery constructed with 12 cells will not fit into a battery compartment sized to accept six cells. Another disadvantage of the xe2x80x9cmonoblocxe2x80x9d approach is that, for applications requiring large capacity batteries, battery size may increase substantially. This large, heavy battery may be difficult to handle thus raising safety concerns for personnel and efficiency concerns for the powered equipment.
One of the solutions to these problems, as shown in the prior art, provides for the prefabrication of individual cells and the placement of individual cells in a preformed compartment in a steel tray assembly. Cell compartments are defined by cell-receiving members (partitions) attached to the tray. This approach is still somewhat limited in that each cell compartment is sized to accept only one or, at the most, two cells. Proper compression for the lead-acid cells is accomplished by limiting the cell compartments to one or two cells and dimensioning the cell compartments to be just slightly larger than the cell dimensions so that the cells may be moved into position without difficulty. Later, in use, the cell walls expand causing proper compression to be applied. Thus, for a tray to contain six cells, at least three, and possibly six, separately formed cell compartments are needed.
Recent research has demonstrated the necessity of initially applying and maintaining modest to high levels of compression in fibrous mat cells to keep the separators in close contact with the plates even before formation and use. Having multiple cell compartments, each dimensioned even slightly larger than the cells to be placed therein, will not achieve this. Further, for applications requiring vertically larger trays, it has been recognized that without partitions or restraints of some type, maintaining the walls of the tray vertical, without any appreciable bulging or flexing, is problematic. This is particularly so when six or more cells are stacked in the tray. The consequences of bulging or flexing are that proper compression on the cells may be lost and the structural integrity of the battery system may be jeopardized during use. One solution has been to increase the thickness of the side walls. While effectively reducing the bulging, this approach requires additional expense and tends to increase the overall width, weight, and cost of the battery assembly.
The present invention relates to a unique and improved housing for valve-regulated lead-acid (VRLA) batteries, and the battery system itself, used in applications requiring xe2x80x9cdeep dischargexe2x80x9d duty cycles.
Specifically, the housing and system of the present invention are directed to deep discharge applications where the individual battery cells are arranged with the longitudinal axes of the cells lying in a horizontal plane, the cells being placed adjacent to and/or atop one another. This significantly improves the cycle life of each cell. Taking advantage of this horizontal orientation of battery cells requires that an appropriate amount of compression be applied to each cell. One approach is to employ various types of compression members within the housing (parent Pat. Ser. No. 6,162,559. However, another approach, and one object of this invention, is to dimension the entire battery housing to receive a desired number of stacks of formed cells where the distance between the housing walls parallel to the cell plates are slightly less than the combined uncompressed height of the stacks of electrolyte filled battery cells. As will be appreciated by those skilled in the art, the cell casings are typically manufactured to have flexible thermoplastic side walls Polypropylene is a commonly used material for this application. When the plates and separators (the plate and separator group) are inserted into the casing, the walls parallel to the plates will bulge. This bulging is the result of the sizing of the fibrous separator thickness such that the inner dimension of the casing is less than the uncompressed dimension of the plate and fibrous separator group. The plate and fibrous separator group must, therefore, be compressed to fit into the casing or jar. This compression is possible due to the sponginess of the separator. Once inside the casing, the plate and separator group exerts an outward force on the casing walls, causing them to bulge. By properly dimensioning the battery housing, when emplaced the casings will resume their original configuration, and the pressure exerted to eliminate the bulge will result in the proper compression on the plates and separators.
In certain configurations, such as those of the present invention, the battery housings are relatively taller, resulting in a greater height to width ratio. With a larger vertical dimension, the thin walls of the battery housing are even more susceptible to bulging after the stacks of battery cells have been compressed. The present invention solves this problem without the need for increasing the wall thickness of the side walls of the housing.
The housing of the present invention is of the type for receiving two or more stacks, each stack comprising at least three valvo-regulated, lead-acid cells, each cell being of the type that includes multiple positive and negative plates with separators therebetween assembled in a case having external terminals. The housing for the stacks of cells includes a base, a top wall, and a pair of side walls. Depending on the configuration, a rear wall with a removable end plate, or a central wall with two removable end plates, may be provided to support the end of cells and enclose the housing. The housing may be formed of steel or other suitable rigid or semi-rigid materials. Dependent upon the height of the battery housing, at least one restraint is provided to extend horizontally between and attach to the side walls of the housing for maintaining the side walls in a substantially vertical position so that they do not appreciably bulge outward and, thus, maintain proper compression on the sides of the battery cells. As used herein, the term xe2x80x9crestraintxe2x80x9d refers to both single-piece members, and to multiple-pieces collectively functioning to maintain the side walls of the battery housing substantially vertical. The restraints are selected and located so that they effectively divide the battery housing into two or more stack-receiving sections. For example, for two stacks of cells, one restraint would be required to divide the housing into two sections. For three stacks, two restraints might be required, etc. As used herein, the term xe2x80x9cdividexe2x80x9d means to separate into sections, but does not mean that the sections must be physically cut-off from one another or that the sections must be equally dimensioned. For example, a housing configured to hold 9 cells could be divided with one section dimensioned to receive 4 cells and another section dimensioned to receive 5 cells. Depending upon the type of restraint use, the sections may or may not be isolated from one another.
A wide range of shapes and materials may be used for forming the restraints of the present invention provided they possess the requisite tensile strength and a low degree of elasticity to hold the side walls in substantially vertical relation. They must also be capable of being welded, bonded, or mechanically attached to the interior side walls of the battery housing. For example, metals, plastics, fabrics, etc. may be used. With respect to geometry, a restraint may be formed as a single continuous sheet or plate, or may be perforated, or slotted. The restraint may also be formed as multiple smaller pieces such as a series of parallel strips, bands, or the like appropriately spaced apart and attached to the side walls such that the cells in contact with the strips are evenly compressed across their horizontal wall surfaces. The restraints may take on these many forms and be constructed of various materials since they are not required to, but may, function as structural shelves or supports for the stacks of cells.
The housing of the present invention may also be further divided into front and rear portions. That is, at least one inner partition, or wall, is so formed that stacks of three or more cells may be placed back to back.
As already described, the battery system of the present invention comprises at least two stacks of horizontally oriented cells, each stack having at least three separately cased cells. Proper dimensioning of the housing and cell-receiving sections formed therein is necessary to ensure that a good initial contact between plates and separators is established when the stacks of cells are assembled in the sections of the battery housing. In general, the battery housing will be dimensioned and appropriate thicknesses chosen for the restraints such that the inside vertical dimension of each cell-receiving section of the battery housing is less than the combined vertical dimension of the stacks of cells in their initially filled and uncompressed state. In an alternative embodiment, the cells are laid on their side so that the longitudinal dimension of the plates are horizontal but the plates rest on their side edge. In this arrangement the inside horizontal dimension of each cell-receiving section is less than the combined horizontal dimension of the stack of cells in their initially filled and uncompressed state.